text:
Could the non-smoking sign still be turned off in modern airplanes?
Yesterday, I flew on Germanwings 4U2379 with an Airbus A319. The plane's interior looked pretty modern, and while I can't find a construction year online, I'm pretty sure it was built way after the total smoking ban.
This plane still had illuminated non-smoking signs though and I was wondering if the crew would actually be able to turn these off during flight if they wanted to. Is there any information about that?
This question shows that some planes do not even have a non-smoking sign anymore, but it does not elaborate on the mechanics of existing signs.

Related.

There is no such thing like "the total smoking ban" as it's still up to the airline / its country if smoking is allowed or not. May be, the illuminated sign is still there to permanently point the passengers to the fact that it's not allowed. (They also don't stop to say that it's prohibited on the toilet, too, and that the toilet has smoke detectors...)

I get why it is still there. But Germanwings has no flights where smoking is allowed and my questions was whether they could still turn it off or if that was prevented by design.

At least here in the U.S. (where smoking on board has been illegal for quite a while,) a lot of the planes now just have permanent placards in lieu of the lighted no smoking signs.

Keeping the sign operational makes it easier to sell the plane in a few years to an airline where smoking is still allowed.

It would appear that the aircraft you flew was D-AGWR, a A319 with MSN (Manufacturer Serial Number) 4285 delivered in April 2010 (hence fairly new).

There is a switch at the bottom of the overhead panel, next to the seat belt sign, slightly right of the middle. Flip it and they will extinguish. The functionality is a bit more complicated than it would appear, since it feeds into the central aircraft electronics.

Olivier Cleynen at Wikicommons
Out of interest, this button no longer exists on the Boeing B787, suggesting that it is on the way out. Only the seat belt option remains.
The slots for no smoking are being increasingly 'redeveloped' into a sign for electronic devices instead on some types. They just put a no smoking sticker on the seat in front of you to fulfil the same function. I do not know if they cockpit text has been modified on the aircraft that undergo this modification.

Source

How did you go from flight number to aircraft serial number?

@cpast http://planefinder.net/data/flight/4U2379 and then via registration to serial number through planespotters.net.

@MikeFoxtrot Thanks for pointing out that link! It might be a good idea to even post cpast's question and (perhaps a slightly expanded version of) your answer as a separate question, since it's rather useful information and that would make it searchable.

The switch still exists, but it's now to light up "Turn off PED", as shown on the image.

@reirab good idea -- posted. I'll let MikeFoxtrot post his answer, as he'll be better able to expand on it than I would.

@mins - true in some aircraft, but not always. Some operators use the PA to advise on PED usage, instead.

What's sad about this, is that the "turn off electronic devices" is already out of date, because now we're always allowed personal electronic devices (though there are times when we can't use laptops)

@McKay In the U.S., yes, but many countries unfortunately still have the electronics ban. Some of them even ban any kind of transmitting device (including even Wi-Fi and Bluetooth) for the whole flight duration. Thankfully, more countries are starting to abandon those rules, though.

In extremis, every electrical item on a plane can be switched off, to protect the plane if the item malfunctions. For example, a short-circuit in the no smoking signs could, in principle, cause a fire. At the very least, there will be a circuit breaker that can be removed to remove power from the signs. However, I don't know if there would be an individual breaker for that system: there might be a single breaker that isolates several related systems.

I was just going to say this myself. Good answer.

Or it could cause an odor, which isn't a good sign either.

In the Airbus A320s I have piloted, no you cannot turn the no smoking signs off even if you put the switch to off position. I am pretty sure, back in the day when smoking was allowed you could do it and there maybe some older MSNs that allows the pilots to switch off the no smoking signs. In later models it is always on. I am not sure what exactly is a later model. I have flown A320s built as early as 2005 and the no smoking cannot be turned off in them.
The reason why there is an off position in the no smoking switch is because in Airbus aircraft it performs some other function as well and that is, it controls the aircraft emergency EXIT lights. You can see these lights in the aircraft ceiling and main entrance and emergency exit doors. From an operational point of view, during cockpit preparation we put the no smoking switch to auto position. When in auto position, with landing gear extended the emergency exit lights come on. Once the landing gear is retracted after take off, the exit lights go off. Anyone who has flown an Airbus would have most probably heard a 'bing' right after take off. This is the chime that comes on when the exit lights are turned off. When you are coming in for landing and when the landing gear is extended again the exit lights are turned on and the distinct 'bing' is heard.

An A320 EXIT light.
Once the flight is done with we do an aircraft securing checklist and this involves turning off the no smoking switch. One might wonder why we do that even though it does not turn off the no smoking signs. The thing is, if the switch is kept in auto or on, the exit lights remains on as long as the aircraft is powered. This prevents the internal emergency light battery packs from charging, because with the lights on, the charging is automatically inhibited. The last thing you want is a drained battery pack in an emergency evacuation. Now, that would not be fun, would it? Panicking passengers in a totally dark aircraft!

One option, used by at least some airlines, is that while the lighted "no smoking" signs have been replaced by placards (no lightbulbs to ever need replacing), the cockpit switch itself still works in that it produces a single tone chime when turned on or off. Since this chime is audibly distinct from the usual high-low F/A Call chime, it could be used as a method of notifying the cabin crew (although I don't know if or how often it's actually employed that way).


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Does the Predator drone's design make a tail strike more likely on takeoff and landing?
I was reading an article about drones and happened to glance at an included 3-view of the General Atomics Predator series UAV. The inverted-V tail appears to leave, at most, around 6" of clearance to the ground.

On takeoff or landing, how is there enough clearance to allow the operator to rotate? Is the wing designed to have a high angle of incidence on the ground?

The tail arrangement was initially chosen to protect the propeller from touching the ground first. The configuration of what later became the Predator was initially designed to be launched from a submarine, so it needed to fit into a torpedo tube when folded. When this configuration was scaled up, the general arrangement was kept. Now the tail surfaces are much larger relative to the propeller, so the initial reason for their location is not easily discernible.
The high aspect ratio $AR$ of the Predator's unswept wing gives it a much higher lift curve slope than that of the swept wing of an airliner, so less rotation is needed to increase lift for take-off. Also, the highly cambered airfoil has a very low, negative zero-lift angle of attack, so at level attitude the wing already produces substantial lift. This was chosen to fly at optimum endurance speed with a horizontal fuselage attitude. The lift coefficient $c_L$ for optimum endurance is $$c_L = \sqrt{3\cdot\pi\cdot AR\cdot\epsilon\cdot c_{D0}}$$
At that lift coefficient, induced drag $c_{Di} = \frac{c_L^2}{\pi\cdot AR\cdot\epsilon}$ is three times bigger than zero-lift drag $c_{D0}$, which means endurance is longest when the aircraft flies at a high lift coefficient and low speed. Basically, the Predator is a point design, flying almost always at the same attitude and lift coefficient. Ground attitude equals flight attitude, and a rotation beyond a few degrees will make it stall.

just a small correction: Predator and other UAV's fly at near-constant CL for optimum endurance = true. But the speed that they fly of course changes with aircraft weight (as fuel is burnt), and endurance UAVs have a high fuel-fraction.

@GürkanÇetin: Right, speed at the end of the mission is maybe 75% of what it was right after take-off. Changed the last paragraph.

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text:
What concerns do the FAA and EASA have in 2014 with "own-ship position display" on EFBs?
I've read a couple of times that "own-ship position display" is not authorized for use on Class 1 or 2 EFBs (by the FAA), and that both the FAA and EASA are cautioning against that feature's use on EFBs in general. Own-ship position is an aircraft's GPS position displayed against georeferenced charts and diagrams.
Obviously the use of a portable device - iPad or otherwise - should not be enough for primary navigation, but what safety concerns do the authorities have with own-ship display?
Specific accident reports or official citations would be great.

Can you clarify exactly what "own-ship position display" actually means? I can guess, but I've never heard the term before and a link to a definition would be great.

@Pondlife I'm actually having a really hard time finding a legit definition. I'll update the question when I've got something; in the meantime, it's basically georeferencing the aircraft's position onto a chart.

Display of ownship's position has a very strong effect on the mental model that the pilot has of it's navigation. When the EFB displayed position is wrong, the change that the pilot will detect that by comparing to other instruments or outside visual are very limited. This is a safety concern.
To have the EFB approved to display ownship's position, the most straightforward way would have been to certify it under TSO C129a "Airborne Supplemental Navigation Equipment using the Global Positioning System (GPS)" TSO-C129a has now been superseded by TSO-C196 which has slightly different requirements.
Often class I or class II EFB will have a GPS receiver that technically could be used as the source of ownship's position. The GPS receivers chips used in are not designed and certified against the "Minimum Operational Performance Standards for Airborne Supplemental Navigation Equipment Using Global Positioning System (GPS)” (RTCA document DO-208), which is a requirement of TSO-C129a or "Minimum Operational Performance Standards for Global Positioning System/Aircraft Base Augmentation System" (RTCA document DO-316) which is a requirement for TSO-C196.
The major technical difference between simple commercially available GPS chips and aviation grade GPS chips is that the latter include a Receiver Autonomous Integrity Monitoring (RAIM) function which does Fault Detection (and Exclusion).
If a GPS satellite would drift out of it's orbit or GPS data gets corrupted in the receiver, an ordinary GPS chip would most likely display a position error, while an aviation GPS chip would either indicate a failure state (fault detection) or still display the correct position (fault exclusion). This protects the pilot from being mislead by a map shift.

Also, in a portable device without an external mounted antenna, it is much more likely to have part of the available satellites unavailable (blocked by part of the airframe) which makes it much more likely to have a position error.

Do you know of any examples where a pilot's inaccurate EFB caused an accident or incident? Or do you think that the FAA is speculating?

I don't know of any particular GPS/EFB accident or incident from the top of my head, but the problem with potential misleading GPS position is real.  Although there is a healthy portion "better safe than sorry" involved in certification, the FAA is certainly not speculating here.

This is part of why I always have Foreflight's "position accuracy" element turned on. If it starts indicating less than 5 meter accuracy you probably don't want to trust it. (That indication itself isn't perfect - certainly not enough to rely on as a primary position source - but it's better than nothing...)

I can't speak for EASA, but as far as the FAA the short version (from AC 120-76B) is that In-flight depiction of own-ship position is classified as a major safety effect and cannot be formally authorized for use on a Class 1 or Class 2 EFB.
So basically the only kind of EFB that the FAA wants displaying own-ship position in flight is a Class 3 system (basically a permanently-installed piece of hardware that's gone through all the certification and testing requirements of any other piece of installed navigation equipment, like for example your panel-mount GPS or an approved MFD like a Garmin GMX 200).
As far as I can find the FAA hasn't given an official reason for this (I went on quite a hunt), but I suspect it is out of an overabundance of caution: Class 3 EFBs are subject to strict design and change controls (like any other TSO'd avionics hardware), whereas iDevices, android tables, laptops, and their internal/plug-in/wireless GPS receivers are not.
The FAA has no way of ensuring that changes (like say a Stratus firmware update or an iOS update) won't adversely affect the accuracy of own-ship position display, nor can they guarantee that the EFB software will correctly detect GPS errors and disable own-ship position display. They would prefer you not have the display at all than rely on possibly inaccurate information and wind up flying through airspace you shouldn't be in (or worse, flying into an obstruction) because you were trusting your EFB's position display.

A tiny bit of light at the end of the rainbow, at least for us light General Aviation piston folks:
AC 120-76B is specifically aimed at part 121, 125, 135, 91F, and 91K operators (i.e. "Not light GA").
The rest of us Part 91 folks are supposed to look at AC 91-78 (which talks about EFBs and removal of paper from the cockpit), and that Advisory Circular is blissfully silent on the subject of own-ship position.
That seems to imply that you can strap your iPad to your Cessna's yoke and turn on own-ship position display and not incur the wrath of the FAA - an interpretation you can find others on the internet agreeing with - though it should certainly not be considered a navigational reference (merely "an aid to situational awareness").
It's also worth noting that if you don't display own-ship position data there's nothing that says you can't use it -- e.g. by turning off the little airplane graphic but still centering your map based on your GPS information. I know ForeFlight has this capability.

Standard Disclaimer:
  I'm not a lawyer and I don't work for the FAA. My interpretation of regulations is worth exactly nothing.

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How thoroughly can WINGS phases substitute for a flight review?
If a pilot is unable to act as PIC because they need a flight review, can they instead earn WINGS credits and complete a phase? Everything I have read, including §61.56 and AC 61-91J, suggests that the answer is "yes", but apparently nobody else is discussing it because I can't seem to find the answer on the internet.

All Wings courses that qualify for a Flight Review include both a online knowledge activites as well as required flight activities that are verified by a CFI. These flight requirements generally end up being longer than the 1 hr minimum flight time required by §61.56.  But if you are completing a Wings course for the purpose of insurance discounts, a BRF would be a side benefit.

To accomplish each Phase of WINGS, you can attend safety seminars,
  take online courses, and demonstrate flight proficiency.

Also, section 1.3 of the Wing Manual states:

Note that pilots need not accomplish the flight review requirements of
  14 CFR part
  61.56, if he or she has satisfactorily completed any Phase in the WINGS - Pilot  Proficiency Program

So, the answer is yes, you can use Wings phases as a substitute for a flight review, but it might not be the most efficient way.

Yes, but you still need a signoff from a CFI.  My group of friends (and my CFI) use wings.  I did that one year but I prefer flying the BFR because that's skill based.  I still do wings credit courses though; you can never learn too much.


text:
For light, GA aircraft, is there a "standard" turn, and what does that mean?
I have seen instructional sites that seem to indicate a 15 degree bank is a standard turn.  The Turn Coordinator has 15 and 30 degree marks on it.  Is 15 degrees a standard turn and what does this mean?  Is there a general way to figure out what the radius of such a turn is or how long it would take to fly a circle at 15 degrees bank?  Without trying to do the math, it seems that airspeed together with bank angle would determine the turn rate.

A turn coordinator does not have "degree" markings, and it does not indicate bank angle.  A functional AI shows bank angle. See also: http://aviation.stackexchange.com/questions/16534/what-is-the-difference-between-turn-coordinator-and-artificial-horizon

Oh yes thank you, I see I was mistaken.  I mistook the rate of turn mark for a bank angle.  I didn't realize the instrument was gyroscopically driven.

The FAA Aeronautical Information Manual defines 'standard rate turn' as,

A turn of three degrees per second.

This results in the completion of a full circle in 2 minutes. Every airplane must be able to complete a standard rate turn in order to be certified. The standard rate of turn is used as the main reference for bank angle.
GA aircraft are expected to turn at the standard rate of turn during holding, intercepting, tracking, approaches, departures etc.
You're correct that the airspeed determines the turn rate. As the turn rate is standardized, the bank angle becomes dependent on the air speed. From FAA-H-8083-15A Instrument Flying Handbook,

True airspeed determines the angle of bank necessary to maintain a standard-rate turn. A rule of thumb to determine the approximate angle of bank required for a standard-rate turn is to divide your airspeed by 10 and add one-half the result. For example, at 60 knots, approximately 9° of bank is required (60 ÷ 10 = 6 + 3 = 9); at 80 knots, approximately 12° of bank is needed for a standard-rate turn.

The part about certification is wrong. Transport aircraft use 3 minute turns, and heavys can use 4.

http://aviation.stackexchange.com/questions/1630/standard-rate-of-turn-for-heavy-aircraft

The part of your question concerning the standard turn rate is already answered by aeroalias, so let me focus on the physics behind it:
In a turn the wing has to produce a centripetal force in addition to the lift $L$ needed to support the aircraft's weight $m\cdot g$, requiring a bank angle $\Phi$. The aircraft flying at a speed $v$ will experience a load factor $n_z$ exceeding unity:
$$n_z = \frac{L}{m\cdot g} = \frac{1}{cos\Phi}$$
The turn radius $R$ is
$$R = \frac{v^2}{g\cdot tan\Phi} = \frac{v^2}{g\cdot\sqrt{n_z^2-1}}$$
and the turn rate (expressed as angular velocity $\Omega$ in radians per second) of that turn is
$$\Omega = \frac{v}{R} = \frac{g\cdot tan\Phi}{v} = \frac{g\cdot\sqrt{n_z^2-1}}{v}$$
Now you can find out which radius and turn rate are flown with $\Phi$ = 15° of bank in a coordinated turn.

Parameters for turns with 15° bank angle (own work). A turn rate of 3° per second can only be flown with 15° of bank at a speed of 50 m/s (97.2 kts). Flying slower will require less bank for the same turn rate or result in a higher turn rate for the same bank angle.


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Are there some minimum criteria for a field to be registered with the FAA as an airstrip?
I spend an unacceptable amount of time 'flying' around the country (U.S.) via Google Earth and I'm always zooming in on airplane symbols to have a look at airfields.  In the western U.S. there are lots of rural strips that have the little aircraft symbol on them which I'm presuming come from the FAA.  Some of these fields appear marginal at best as legitimate airfields, especially the ones that are obviously part of a farm or ranch.
My question is what level of improvement is necessary for a field to be listed by the FAA as an airstrip?  Is there some minimum length, width, slope, etc.?  Is there some inspection that is done before you can get put on the charts and are there any ongoing checks to make sure the field is still there when the charts are updated?  Does the owner of a strip assume some ongoing obligation as a result of their strip showing up on a sectional?

Check out the airstrips in the Australian outback sometime. "Marginal" would be a generous description of some of those!

My guess is that it has to receive the Ben Franklin certificate of approval.

What may appear "marginal" to someone who's used to the facilities of a major airport is often not so marginal after all for the planes that use it -- cropdusters and small trainer planes don't need a ton of room to take off and land, nor do they need extensive ground support (a 55gal drum of avgas with a fuel pump on top is plenty).

You might also check out this question for the unexpected benefits of having your airstrip registered

It looks like the current answer could to be selected.


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Do propellers have any lead/lag mechanism?
The fan blades in a fanjet are mounted loosely in a fir-tree shaped mount so they are free to lead or lag and balance themselves. The blades on fully articulated and semi-rigid helicopter rotor heads are mounted with a drag hinge that allows them to lead/lag as needed. Do any propeller aircraft have a method to allow the blades to lead/lag, or is this for some reason unnecessary?

Are you talking about GA aircraft or something bigger like a turboprop?

@RonBeyer Either. Just wondering if this principle is ever used for propellers at all

The two things are completely different and exist for different reasons:

Fan and turbine blades are mounted loosely, so they can self-balance. This is because at the very high RPM fans and turbines spin at they are extremely sensitive to imbalance. On the other hand, the self-balancing only works at high RPM.
Rotors have lead-lag hinges because the rotor spins parallel to the flow so in forward flight the advancing blade  meets the air at significantly higher speed than the retreating blade.

Propellers spin slower, so they are not as sensitive to imbalance, and they don't have the shroud, so they can't be mounted loosely anyway. They spin perpendicular to the airflow, so all blades meet the air at similar speed and angle of attack, so they don't need to deal with as big differences in the aerodynamic forces. And they are smaller and more rigid.

Re "It also requires the blades to be connected at the tips with the “shroud"" -- what does this mean?  The blade tips are physically scraping on the shroud?

@quietflyer no; some have connecting links on the outer edge. But I think it was actually a braino—removing.

I don't think props do (usually) have lead-lag hinges. Those lead-lag hinges are only necessary as a complement of the flapping articulations in fully articulated rotors, as in many helicopters, and their role is to accommodate the periodic variations in angular velocity of the blades, since the tip-path blade orbit is always a circumference, but when that tip-path plane is at an angle w.r.t. the plane perpendicular to the engine shaft, its projection is an ellipse, and the resulting periodic variation in blade angular speed has to be 'accommodated' with the led-lag hinges.
When a rotor (or a propeller) has no flapping hinges, there's no need of lead-lag articulations...

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Where did the "K" identifier prefix for US airports come from?
Airports in the United States are (mostly) three letters, prefixed with a K. Where did this standard originate? Why K?
This is similar to "where did the U.S. N registration letter come from", but different!

I suspect it originally comes from the K prefix used for radio stations west of the Mississippi, but I don't have any solid references to back that up (or why airports east of the Mississippi also use K and not W).

Be aware that not all U.S. airports use K. Alaska, at least Anchorage, uses P as does Honolulu. My guess is that all Pacific region U.S. airports use P.

P is for Pacific, and if you're wondering who uses the U, that would be Russia. :)

The letter K was simply assigned to the contiguous US by ICAO, in order to have a system with unique identifiers for world-wide use, instead of trying to adapt local system to match. The IATA codes had been in use already and possible duplicates could not be excluded. The K in US ICAO codes is there to mark the region the codes is in, see the chart below.

A good summary of how many of the airport codes have been created can be found on www.skygod.com.

Airlines use the three-letter codes internationally in their own network, Sita, for messages such as passenger loads and departure times. World ATC and weather agencies use a separate teleprinter network, the Aeronautical Fixed Telecommunications Network (AFTN), which uses a four-letter "location indicator." Going from large area to actual airport, the first letter relates to the part of the world and the second letter the country. The third letter is a group of airports within that country. Most countries who use this particular convention use a letter to denote the FIR in which the airport is located. So F is Frankfurt FIR in Germany, M is Munich; P is Paris FIR, M is Marseilles. Other ways to use the third letter include identifying a group of airports with a common factor. For example, A was used in Germany for all Canadian and American air force bases. The last letter positively identifies a specific airport.
Thus Aberdeen, Scotland, has the International Civil Aviation Organization (ICAO) location indicator of EGPD—E for Northern Europe, G for United Kingdom, P for Scottish region, and D for Dyce field. Want to figure out LFPG? It's L for southern Europe, F for France, P for Paris FIR, and G for Charles de Gaulle airport. Easy!  Some are more easy to remember. The Dominican Republic codes all start with M (Central America) and D (Dominican) and then, well, see if you spot the code for international airports in Santo Domingo (MDSD), La Romana (MDLR), Puerto Plata (MDPP), Punta Cana (MDPC) and Santiago (MDST).
So if London Heathrow has two codes — and it does, LHR and EGLL — how come I've heard Chicago O'Hare only called ORD? The answer is unique to the United States and Canada. In the 48 contiguous States the ICAO code is formed simply by adding a "K" to the three letter code (or by adding a "C" in Canada). This explains why international flight plans refer to KORD, KMIA, KJFK, etc. A meeting of two rules is Key West, the FAA code is EYW (lose the 'K') and the IATA code is KEYW (add a 'K') which works great for KEY West.
Other airports within FAA jurisdiction have ICAO codes usually formed by taking two letters from the FAA name and prefixing them with PA for Alaska, PH for Hawaii, or PG for Guam. PT appears to be Pacific Trust Territories (Pohnpei, Yap, Chuuk, etc.), and there are various random other P codes like PWAK for Wake Island.
While most U.S. airports have the same three-letter location identifier for the FAA and IATA, note I didn't say all do. Scenic Sedona Airport in Arizona is assigned SEZ by the FAA and SDX by the IATA. Because for international airlines SEZ is Seychelles International Airport in Mah�, Seychelles. Another is the Hilton Head airport in South Carolina mentioned earlier. It's IATA code is HHH, but it's FAA three letter code is HXD.

Related question:
What is the methodology for assigning airport codes?


text:
What were the typical dimensions of a WW2 US bomber formation?
The idea behind daylight raids was that bombers were more accurate when they could actually see targets so the damage inflicted would be concentrated more so that a night time carpet bombing.*
All of the footage I see, including CGI in movies, depicts large formations of US bombers giving the impression that the "precision" raids were anything but precision.
What was the "footprint" of an average American daylight raid?  If you had to draw a 2D box on a map, what values would you assign to length and width?
*Not saying that it was done to protect civilians, clearly the objective was to destroy strategic targets.

Footprint meaning the size of the aircraft formation, or the dimensions of the bombed area?

Interesting.  I assumed they would roughly match since winds aloft would have similar effects on each individual bomb.  If I have to choose, I'm more interested in the formation itself.  That said, it would be interesting to compare both.

Not necessarily, it might if they all dropped at the same time, but they usually have specific targets so at the very least it would be as wide as the formation, but doesn't have to be very long.

Right, the idea being to land bombs on a specific target.

Aside from a few specialised squadrons and weapons, there was no such thing as precision bombing in WWII.  Googling will quickly find statistics but I seem to recall that there was less than a 1% chance that a bomb would fall on target (within 1000 feet).  Most factories either did not stop production or were back online within a day or so. There was a raid on a Japanese plant with hundreds of bombers that put a single bomb on target. I would say that "many square miles" is probably as good an answer as you could get.

The size of the bomber formations (bombers streams) varied significantly depending on the number of bombers , the time , location and other things. The most common figure quoted figure is around 70 miles long and  5-6 miles wide (for ~500-600 bombers). However, the formation's footprint depended on a number of factors ranging from the navigator's skill to the wind in that area.
The figures vary wildly. For example, from the book Mission to Berlin: The American Airmen Who Struck the Heart of Hitler's Reich:

The stream of more than one thousand Eighth Air Force bombers, from one end to another was 360 miles long. ... on ... February 3, 1945, when the first flying fortresses reached Berlin, the last bomber was over the Zuiderzee in Holland.

This extreme length is a consequence of the situation late in the war, where allied air armada roamed European skies without bothering much about the Luftwaffe. The time spent over the target, on the other hand was quite small-

... each Fortress crew would spend only between thirty and sixty seconds over the center of Berlin itself...

Note that there was no such thing as precision bombing in WWII (except for some special cases), though the accuracy did improve over the war. Of course, some of this was due to the way the 'target' was designated- anywhere 1000' of the aiming point was good enough. From The United States Strategic Bombing Survey:

Conventionally the air forces designated as "the target area" a circle having a radius of 1000 feet around the aiming point of attack. While accuracy improved during the war, Survey studies show that, in the over-all, only about 20% of the bombs aimed at precision targets fell within this target area. A peak accuracy of 70% was reached for the month of February 1945.

Accuracy of RAF night bombing improved from ~20% in 1942 to > 90% in 1945. Of course, the British target was comparatively larger- anything within a 3 miles radius of the aiming point was a 'hit'.


text:
What provides the greatest thrust in a high-bypass turbofan engine?
I'll try to make as much sense as I can with this question.
I've been flying quite often in 2015, and I have recently wondered this question regarding high-bypass turbofan engines. I know that there are two primary parts to the high-bypass turbofan: the fans themselves that pull air from the front of the aircraft through the engine, and the fraction of that air that gets compressed and undergoes combustion inside the combustion chamber.
Given those two main parts that I know about (or so I think I know about), which provides the greatest amount of thrust for the aircraft, the suction and escape of non-combusted air out of the back, or the hot combusted air that is ejected out of the back of the engine?
Thanks for reading, and I hope this question makes sense.

Related

I analyzed a Pratt & Whitney F100 turbofan last semester in my aerothermodynamics course, so allow me to answer this question. The short answer: the un-compressed air provides the majority of an engine's total thrust since the compressed air powers the engine.
Correction: I forgot to mention that the fans also compress entering air. That is, all air entering an engine is compressed a bit by the fan. Some of this compressed air enters the turbojet core and the rest of the fan-compressed air bypasses the engine core. I ignored this for the sake of simplicity, but I should have explained this since you directly asked about air compression in a turbofan.
Long answer: see below!

1. The fan(s)
Air enters the engine through a fan (or fans in the case of the F100 engine). These are the giant fans with a spinning insignia in the middle that you see inside an engine.
Update: The spinning insignia clearly show if the fan is spinning so workers don't get injured.
The fan(s) increases the pressure of the air that enters an engine. Some of this compressed air is diverted around the rest of the engine and is directed straight out of the engine. The bypass ratio is a measure of how much air bypasses the "jet" core (bypass air/core air).
2. The compressors
The rest of this air is then compressed through a combination of more fans and a converging duct. The data from my thermo project tells me that this stage increases the pressure of the core air by more than 10,000%, but I'm not too sure about that*. Sufficed to say, this core air now has a lot of energy--let's add some more :D
Quick note: The compressed air now has insignificant velocity relative to the bypass air. Most of the compressed air's energy is "in" its pressure (senior SE members, please correct me if I'm wrong).
3. The combustion chamber (aka, combustor aka magic chamber)
Now the core air enters the combustion chamber. Here the air enters small chambers, mixes with jet fuel, and is ignited. The main parts-of-an-engine diagram I posted makes it seam like the combustion chamber is one big part of a jet-engine, but really the combustor consists of a bunch of smaller chambers surrounded the main shaft of the engine. Here is a gif that shows what I mean:

How the combustion chamber operates is beyond my scope of understanding, but consider that a combustor is essentially trying to keep a candle alight in the middle of a hurricane. Awesome engineering goes into designing better and more efficient (hotter-burning) chambers.
4. The turbine (aka more magic section)
Now that hot and even more energetic air enters a the turbine section which consists of a diverging (increasing in area) duct and more fans. Whereas the compress "inserted" energy into the air, the turbines draw out energy from the air. As the air enters the larger (in volume) turbine area, it expands and spins the turbine fans which power the compressors and the fan. This Back Work Ratio (BWR) is a measure of how much turbine power it takes to spin the compressors.
5. The nozzle
The still energetic core air is once again concentrated before being shot out of the back of the engine.This thrust, together with the thrust of the bypass air, propels the air forward following this model:
$F_{thrust} = \dot{m}_{bypass} \times \Delta v_{bypass} + \dot{m}_{core} \times \Delta v_{core}$
Where $\dot{m}_{bypass}$ is the mass flow-rate of air that is bypassed and $\Delta v_{bypass}$ is the change in velocity of that air as a result of the fan.
And $\dot{m}_{core}$ is the mass flow-rate of air that is combusted and $\Delta v_{core}$ is the change in velocity of the core air as a result of the fan, compressor, combustion chamber, and turbine.
The uncompressed air contributes about 60% of the total thrust. The "processed" air loses a significant portion of its energy to powering the engine. However, the compressed air still provides about 40% of the total thrust. Adding an afterburner can increase this contribution to 50%. How is this possible? Dead algae from a billion years ago.
The hydrocarbons in jet fuel pack a lot of energy into a small space and mass (two completely different concepts). Burning those hydrocarbons releases a lot of energy that powers the fan, compressors, and electric generators of an aircraft before pushing the airplane forward. This high energy in a small place/space is also why electric cars/anything weren't practical until LiPo batteries (story for another article).
I applaud you for noticing that the uncompressed air contributes to an engine's thrust. I think the term "bypass" confuses some people into thinking that this air is "thrown away". it's not. The bypassed air is actually sped up by a series of fans and imparts forward momentum to the aircraft.
I'm studying Aerospace engineering right now (Whoop!), so my excitement at being able to answer this question rather side-tracked me from your original question, but I hope you enjoy the extra info on how a turbofan operates.
Additional info
The hotter a combustion chamber burns, the hotter the core air gets and the more energy it can provide for engine operation. Furthermore, hotter chambers waste less jet fuel and result in relatively cleaner and safer fumes. Increasing CC max temps and designing the rest of the engine (i.e., the turbine blades) to handle the increase in temp is the cutting edge of engine research. This is one of the most challenging and lucrative materials science/engineering problems in the world since efficiency is paramount in today and tomorrow's aviation industry.
Here is a nice article describing how GE, Rolls-Royce, and other engine companies sell thrust, not engines. It's a big long, so you may  read it on your next flight :)

You may want to go over this again for grammar and structure, but overall a good answer :).

The turbine spinning direction looks wrong. Also, all of the air is compressed, the bypass air less, the core air more. But the rest is correct, so you may remove the caveats.

I can't do anything about the direction of the turbine blades in the pictures (not mine), but I've corrected the rest of my mistakes. Thanks!

The spinning insignia shows which way no, as @jjack said, it shows IF, "which way" is non-useful information.

@Federico: Oh, important distinction!


text:
Why aren't there simplified fly-by-wire helicopter controls?
I was discussing helicopter control with relatives over the holidays, particularly regarding how difficult it is to fly one and how changing any control requires adjusting the other two main controls. One of my relations asked whether any helicopters have a computer control system that automatically compensates, allowing simplified controls that almost anyone could fly.
I replied that I had not heard of such a thing and could only guess why not:

Reliability concerns and possibly pilot training concerns (i.e., you'd have to know how to really fly in the event of a computer failure anyway, so why even have the computer?)
Subtle complexities of the control interactions not lending themselves to reasonable computer automation.

Are there actually no systems like this? Why not? Were my guesses any good?
Regarding the possible duplicate, I saw that question and it seems to indicate that automatic simplified controls would be possible, but it doesn't seem to fully answer the question of why not have a psuedo autopilot that allows a pilot to just push forward on the cyclic to go forward (for example) without having to constantly adjust the collective and rudder at the same time.

Possible duplicate of Do rotorcrafts like helicopters have autopilots?

I would say it's absolutely possible - look at small scale (RC) helicopters and multi-rotors which do this exact thing. As to why it's not in use, you mention some good reasons (safety, training), and I'm curious what the experts around here say :)

I've never heard of any such system, the problem being that it's a lot more complex that that.  How would such a system know that pushing the cyclic forwards means that I want to go forward?  Pushing the cyclic forward can result in many flight regime changes, depending on what I also do with the power and pedals.  For example, if I push cyclic forward in the cruise, what result do I want?  Accelerate?  Climb with constant speed?  Descend with constant speed?  Climb and accelerate?  Enter an accelerating left hand turn? etc.etc.  An autopilot is simple.  Manipulating the other controls is not.

@Steve The original conversation that bought this question up started on the subject of quadcoptor drones, which as I understand are easier to control specifically because of their quadcopter configuration (which would probably be impractical for a full-scale helicopter and I think that question has been asked and answered here).

@Simon Your comment seems like a good answer to me.

Helicopter manufacturers seem to have been slow to adopt fly-by-wire systems. I used to work in the General Dynamics group which designed the fly-by-wire system for the F-16. That was the first production aircraft which had full-authority fly-by-wire (without mechanical backup systems). By the time I started working there, they were in process of updating the system to incorporate a digital flight control computer. The production required extensive coordination between multiple disciplines. As an electrical engineer, I regularly interacted with software engineers, control-law designers, test engineers, and "mechanization" engineers. It took many years for the company to develop such a culture, where each member is concerned with the efficiency and safety of the aircraft. I speculate the helicopter manufacturers didn't catch up for a couple of decades because funding was limited.
Here is an article which describes the Army's experience: Army Slow To Adapt Fly-by-Wire Controls for Helicopters
Funding aside, helicopters are ideal candidates for fly-by-wire systems. The obvious goal is to take a marginally controllable aircraft and make it both stable and maneuverable. That would dramatically reduce pilot workload. Most helicopter fly-by-wire systems accomplish hands-free hover and decoupled control inputs (i.e. sideways flight without needing rudder inputs).
The first helicopter which demonstrated a great fly-by-wire system was the Sikorsky RAH-66. It even achieved sideways flight at 60 knots and backward flight at 25 knots, using only cyclic stick inputs. Unfortunately, that program was cancelled in 2004. The first production military fly-by-wire helicopter is the NHIndustries NH90, which was the first helicopter to fly without mechanical backup controls, in 2002, and entered service in 2007. The first civilian fly-by-wire helicopter, the Bell 525 is currently under development.

A-ha! So the answer to the question, "why isn't anyone doing this?" Turns out to be, "some people are!"

Yes. I'm excited about the possibilities. If we consider the Boeing V-22 Osprey tiltrotor as an example - the aircraft is too unstable to fly without fly-by-wire. Similar concepts were tried during the 1950s, and they were unsuccessful because the electronics technology wasn't available then. Now, this genre of aircraft is usable, due to fly-by-wire systems.  http://www.verticalmag.com/features/features_article/20112-flying-the-v-22.html

The 525 had a rotor head that went divergent.  I'd be careful about touting its virtues until the NTSB and Bell investigation results are done and Bell proceeds where they left of when it destroyed itself in flight during a test flight.  As to the Army being careful to adapt, there's a good reason for that.  Don't fix what's not broken until what comes after is proven better. With over 2000 Blackhawks in the fleet, the "need" to rush to FBW is NOT demonstrated.  As far as I can tell, the Valor (V-280), the Raider (S-97) and the Defiant are all FBW.  Comanche was but it died during acquisition.

Ideal candidates?  Hardly.  Fixed wing control coupling is far less complex than in rotary wing. That said, there are Rotary Wing FBW suites.  As I noted above, and as you noted, Comanche (RAH-66) was FBW and was going along fine until the program went belly up in the early 00's.  As to hands free hover, I could do that in an SH-60B in 1989 without any FBW.  The AFCS and SAS were robust enough to do that in the coupled approach mode. (OK, technically the horizontal stab was crude FBW in the Seahawk ...)

There are two extremes:

No augmented controls, i.e. most normal aircraft. Pilot controls control surfaces and engines directly.
Heavily augmented controls (left, right, up down shifts exactly like that), like quadcopters. Pilot gives command how the aircraft should move that is translated by algorithm into control surface and engine inputs.

At either of the two extremes, stuff is decently well understood. It's when you go in between that it starts becoming problematic.
Augmented Controls
There is always a conflict between a beneficial change making flying easier for the pilot and a greater level of abstraction from the flight controls. One place where this is particularly prevalent is on Airbus aircraft, where misunderstandings between the pilot, interface, control and aircraft have lead to multiple accidents.
Simon's point here- what do you 'want' you control surfaces to do- is very relevant.
Dynamic System
Helicopters are incredibly dynamic. For instance, a sideways gust will push the helicopter to the side, but that would also result in a disproportionate rotation on the whole airframe caused by the surface area of the tail rotor fin. Yet the pure sideways (not moment) force from increasing the tail rotor power is uncompensated and would require main rotor adjustment again to stay in position. Any pilot response and input would have to be handled appropriately to prevent pilot induced oscillation.
Stability Augmentation is used, for instance on the EC135:

The Auto Flight System is hierarchical in concept and on G-IWRC comprised a three axis Stability Augmentation System (SAS) and an autopilot. The SAS consisted of a  Pitch  and  Roll  SAS  (P&R  SAS)  and yaw  SAS.   The helicopter  was  also  equipped  with  a  pitch  damper.  These systems are used for stabilising the attitude of the helicopter  about  the  longitudinal,  lateral  and  yaw  axes by applying limited authority inputs to the main controls.
The  SAS  system  is  designed  for  ‘hands-on’  operation, which means that the pilot must provide control inputs through  the  cyclic  control  and  yaw  pedals  in  order to  control  the  attitude  of  the helicopter.    The  SAS  is automatically  activated  during  the  start  procedures  and can be disengaged by pressing either of the SAS DCPL
switches  located  on  top  of  each  cyclic  stick  grip. Re-engagement of the SAS is through a four-way switch on the cyclic grip, labelled P&R/P – P/y RST. Eurcopter EC135 Accident Report

Pilot Role and Regulatory Aspects
I've worked on UAV systems that go 'the whole way' on control automation. I think part of the problem is the conservative opinion on the role of the pilot. Airbus have become heavily criticized for their design choices that are supposed to make it easier for the pilot to fly. Augmenting flight controls is risky business.
Changes are difficult to implement and testing expensive in the aviation sector. We could ultimately remove the pilot and whoever onboard (doctor, nurse) could just select a spot on a map as a landing site. By going to the stage of making push up/down/left/right... controls as you suggest, we have created fully automatic flight controls. It is (entirely) technically feasible, since that's what the autopilot does. Changing how active control inputs are interpreted from a use, licensing, training and execution standpoint is a whole different story.

I'm not sure I understand all of this answer. The UAV systems you've worked on are rotor craft or fixed wing?

@ToddWilcox rotor craft

Just a guess, but have you worked the unmanned Kmax?

Yes fly by wire controls are applied in helicopters. But there is a simpler and much older solution to the multidimensional flight control dance in helicopters: the mechanical mixer.
Consider a helicopter in a stable hover. Only small movements around the neutral point of the flight controls are required to compensate for small variations in air flow. Now pull on the collective to gain height, and the neutral point of both the pedals and the lateral stick change: rotor supplies more lift but also more torque, compensate for this with increased tail rotor thrust, compensate for increased sway with the lateral stick.
The mechanical mixer does just that, automatically. It is found in large helicopters like CH-53, UH-60 etc, and is located behind the flight controls but before the swash plate actuators, and it is just a set of mechanical arms and legs that interconnect collective with pedals linkage etc.
Note that the mechanical mixer only changes the trim point of the flight controls, but not the static stability of the helicopter or the cross-coupling effects of rotor inertia etc. For that you would need F-16-like fly-by-wire with redundant computers & all.

To be fair, the UH-60 has a number of roll to yaw coupling inputs to the AFCS (IIRC the signals come from the lateral accellerometers in the  nose compartment) that are not strictly mechanical mixing, right? ;-)

Yes indeed, the autopilot is another means for making flying a helicopter a bit more straightforward :)

The concept of advanced controls for rotary wing craft has been around a long time. I worked for McDonnell Aircraft in their simulation group during the 80's when they were competing for the LHX program (Light Helicopter eXperimental). While I can't say this is the earliest implementation of such controls, it has to be one of the earliest.
The flight controls basically removed the cyclic and collective and replaced it with a four-way sidearm controller -- basically a 3D joystick with rotation. Any horizontal motion within the plane of flight was controlled by pushing the controller in the direction you wanted the craft to move, forward, backwards, or sideways. Vertical flight was controlled by applying upward or downward pressure on the controller. Platform yaw was performed by twisting the controller. --Reference
Though I've loved aircraft for many years, and I've not (yet) gotten my PPL, I used to fly the fixed wing simulators and had fun. But having never flown a traditionally controlled helicopter, I found flying the LHX very intuitive.

I'm sorry, but while this adds interesting historical information, I currently don't see how it answers the "why is not common" part of the question

On main consequence of having independent controls in a helicopter is that the same control movement will have different outcome depending on the movement of other controls.
For example, consider the collective- in hover, varying the collective will result in a change in altitude, while in forward flight, the same thing will result in either a change in altitude or speed depending on how the cyclic is moved. The operation of cyclic has similar results based on operation of other controls.
This is the problem in having pseudo or coupled controls- the helicopter response may differ from that of the intended one. On the other hand, the FBW computer is pretty straightforward- you input the controls, which are sent to the actuators through computer. The computer need not guess what you want to do here.
I think there are some helicopters where moving collective automatically adjusts the torque, though this is the only case where I think the controls are coupled.

Do you mean the fuel governor automatically adjusting engine power upon moving the collective?

Short answer: We already do have Fly-By-Wire rotor-craft from Bell/Sikorsky, Airbus, and probably others. Reference the Bell 525, Sikorsky V22, Airbus/Eurocopter NH90
Long answer: It is hugely expensive and time consuming to get a new aircraft or system certificated. Even simple changes to a certificated aircraft or systems can be hugely expensive and time consuming from a regulatory compliance prospective, which must include testing said aircraft and systems during the development cycle while documenting every tiny detail.
People are resistant to change in general -- making changes to something they know, love, respect, and sometimes fear will bring out the worst in them. Pilots are just people with feelings, desires, and opinions just like everyone else.
At the time of this writing in late 2018, Fly-By-Wire systems are not just technically feasible, they are mature. The electronics and servos are so inexpensive that triple-redundancy is a trivial additional cost. All of our most complex commercial and military aircraft today (2018) have Fly-By-Wire systems including rotor-craft and space systems.


text:
Why evacuate upwind?
Evacuation procedures after a forced landing recommend directing passengers upwind of the aircraft.
What is the reason why evacuating upwind is better than other directions?

Which way will the smoke and fire blow?

There are two problems linked to the wind after accidents:

Inflating the slides.
Running away from the aircraft when on the ground

@DavidRicherby listed the reasons related to running away upwind to try to avoid the effects of flames and fumes (visibility, heat, toxicity). This is part of the IATA guidelines for post-evacuation:

Post-evacuation. Once outside the aircraft, the cabin crew is
  responsible for the passengers until relieved by the authorities or
  emergency services. Until this help arrives, the cabin crew should:
  - Direct passengers upwind and away from the aircraft;
  - Assemble passengers;
  - Direct passengers away from fuel, fire and vehicles;

Source
While it appears running upwind is a good choice, deploying slides upwind has not always been a good idea.
A study was conducted by the National Aerospace Laboratory (NLR) in Netherlands, about slides issues during evacuation. It was published by the Flight Safety Foundation in 2005:

Analysis of Evacuation-slide Problems
Calls Attention to Recurrent Issues

The scope of the search was the period 1970–2003 and covered aircraft operations worldwide. NLR researchers identified 150 accidents in which slides were used. A total of 89 slide problems were identified:

One problem was slides not inflating due to the wind:

Wind had an adverse effect on the use of slides in 11 slide problems
  (12.4 percent) analyzed by NLR researchers. Typically, wind blew the
  slide against the side of the aircraft, preventing its use.

To allow using the upwind side of the cabin for slide deployment, today, aircraft certification include this requirement for slides:

4.20 Wind. The device must be shown, in 25-knot winds directed from the most critical angle, to deploy and, with the assistance of only
  one person who has evacuated down the device, to remain usable after
  full deployment to evacuate occupants safely to the ground.

Source
See also:

IATA - Cabin Operations Safety - Best Practices Guide
EASA ETSO-C69, Emergency evacuation slides, ramps, ramp/slides, and slide/rafts

If there is a fire, any smoke and flames will blow downwind. If there is a fuel leak, the fuel vapours will be blown downwind, risking a fire or explosion there and potentially making it hard to breathe.

The evacuation at least 500 meters upwind is to prevent from being intoxicated with toxic fumes, gases from burning materials and alike

This answer adds nothing significant to the existing answers.


text:
Other aircraft doesn't apply right of way rules
I was flying VFR a couple of days ago and was warned by ATC that another aircraft was coming from my left to my right.
At some point I saw it and we were clearly converging and getting pretty close.
According to the right of way rules I had priority, but the other aircraft didn't change its heading (maybe the other pilot didn't see me - he/she didn't seem to be in contact with ATC).
According to the rules I was supposed to keep flying the same heading and altitude but I decided to turn left to avoid her and it worked fine - however I can see how it could have been dangerous had the other aircraft changed its heading at the same time.
So I'm now wondering:
What is the "right thing" to do when another aircraft doesn't follow the right of way rules?

See and avoid is the backbone of VFR flight. The right thing to do when there is a potential collision is to take positive action, which is exactly what you did. There's no one-size-fits-all solution as to whether to turn one way or the other, climb or descend - it all depends on the position and course of the aircraft in question.
Often which way to turn in these situations is the lesser of two evils, if you turn left you clear sooner but if the other airplane turns right there's a potential conflict, if you turn right then you are in potential conflict longer. You have to choose one and hope the other person takes the right action.
One way to avoid conflict is a change of altitude, if you are slightly higher than the other airplane then climbing quickly takes you out of conflict, if you are lower then a descent does the same thing. If you are the same altitude then I favor descending as most people's default reaction is to climb - if the other pilot sees you they're more likely to pull up, giving you better separation. Even if you are under ATC direction changing altitude is the right action as it's necessary for the safety of the flight. You change altitude, avoid the collision, then go back to your assigned altitude before radioing ATC and explaining the deviation. I doubt they'd have a problem with that and even if they did regulations are on your side.

How are the "right of way" rules worded for aviation? On the water there is no right of way. Instead, there is only obligation to give way. If one boat fails to yield in due time, the course holding boat is obliged to yield. Is it the same in the air?

They are clear, are laid  out in regulation and use words such as "shall" and "must".  They are not optional.

You did the right thing. The opening paragraph of section 3.2 (Avoidance of collisions) in ICAO Annex 2 (Rules of the Air) states:

Nothing in these rules shall relieve the pilot-in-command of an aircraft from the responsibility of taking such action, including collision avoidance manoeuvres based on resolution advisories provided by ACAS equipment, as will best avert collision.


text:
How much does the swirl of engine exhaust affect aircraft performance?
Most multi-engine aircraft have engines that rotate the same direction on both sides of the fuselage (e.g. the GEnx rotates clockwise, as viewed from the front, there is no 'mirrored' version).
What would the effect (drag, stability, turning performance) be, if an aircraft were to have oppositely-turning engines on each side of the aircraft (especially if the engines were rotating counter to the wingtip vortices)?
Note that propeller and jet engines may have significantly different answers!

See https://aviation.stackexchange.com/questions/20735/does-the-direction-of-counter-rotating-props-affect-efficiency

For turbine-engine, flows are mostly straightened by vanes at the level of the fan/rear frames. See In a turbofan what holds the spinning axis?

More of a comment than a complete answer, but  the losses caused by swirl are not really on drag, stability, and turning performance. The swirl represents is a loss in total pressure in the axial direction, and so is a loss in shaft power extracted by the power turbine, or thrust provided by the engine if it’s a turbofan. This won’t reduce drag at the same aircraft velocity one iota, but does enable you to go faster if you reduce the swirl, due to the higher power (in a turboshaft) or thrust (in a turbofan). How much effect swirl has, I am not sure. I think it’s pretty small, because some engines have vanes at the turbine exit to remove, and others don’t bother, and instead choose to save the weight of these vanes.
Fairly obviously, extra power or thrust will help the vehicles turning performance, but I think it’s clearer to state this is due to extra power or thrust, rather than imply it’s a direct effect.


text:
Can a laser Doppler anemometer be used to measure airspeed?
Could this be used in place of or in addition to a pitot-static system?

In place of pitot-static system
No, because it measures different physical quantity.
Laser Doppler anemometer measures velocity of the stream¹. But pitot-static system does not. Pitot-static system measures dynamic pressure and expresses it as airspeed at which that dynamic pressure would occur at sea level on an ISO standard day².
Since lift and control effectiveness depend on dynamic pressure, that is exactly the quantity the pilot needs to see. Laser Doppler anemometer does not provide it, therefore it cannot replace pitot-static system.
In addition to pitot-static system
Anything can be used in addition to what we have. But the question is whether there is a reason to.
For flying the aircraft, true airspeed is not needed and for determining the wind speed by comparing true airspeed and ground speed the airspeed can be estimated well enough from indicated airspeed, pressure and temperature.
However a laser anemometer has the advantage of being able to measure the wind speed ahead of the aircraft. This could be used for load alleviation (the system needs to know about the gusts a bit in advance so it can move the control surfaces in advance) and it could perhaps be used for detecting wind shear, though I am not sure how feasible such anemometer measuring wind speed hundreds of metres ahead is.

¹ A.k.a. true airspeed.
² That quantity is called equivalent airspeed. That is intended quantity, but the measurement is subject to various errors. The raw output of the pitot tube is called indicated airspeed and the value corrected for positional errors (due to the air-stream impacting the pitot tube at an angle) is called calibrated airspeed.


text:
Does anybody recognize the biplane used here?
Little red biplane used here in WKRP In Cincinnati S3E01.  I don’t recognize the type and I’ve never seen this aircraft before. Of interest appears to have trailing link main landing gear.  Anybody know what type of airplane this is?

I wonder if there's any chance that it's a "crosswind" gear set-up?

The type of airplane is in the description of the video.

It was N14041 which was a Waco UMF owned and flown by Harold Johnson.
Aerial Visuals N14041

Waco umf3 the flying mayor of Moraine Ohio, Harold Johnson and it now sits in the Waco museum in Troy Ohio. I grew up flying with Harold his hanger and my father's were connected back to back with a door between them. That's Darrell's (my father) 450 stearman in the background of the 2nd picture.

Hi Brian, and welcome to Aviation.SE.  That's very cool that you have the personal connection to that aircraft from younger days!  Thanks for adding the additional info on the aircraft's current location - no doubt it's a beautiful part of their collection.

Great info, but it should really be a comment on the other answer. Once you have enough rep, you'll be able to comment on anyone's question or answer.

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review


text:
On crosswind landings, is it preferable to side slip or crab on final approach?
I've seen several instructional videos on crosswind landing techniques.  Some advise the pilot to crab the aircraft until just before touchdown when he/she should kick the rudder over to align the aircraft with the runway centerline.  Others advise the pilot to slip the aircraft all the way down so that no other cross control maneuver is needed.  Many of these videos also acknowledge the other way of doing things but say that their method is superior.
Is there an accepted "safer" method for flying a final approach with a crosswind or is it an entirely personal preference?

Well so far the answers parallel the question, i.e., good descriptions of both techniques and justifications for each of them.  I have always felt better crabbing on final until I read about some CFI's really emphasizing a stabilized approach and using that as a justification for slipping on final.

It can depend on the type of the aircraft. Aircraft with very long and low wings (e.g. most gliders) cannot land in a slip as they would hit the ground with the wingtips first.

Doesn't sliping reduce margins over stall ? It always seemed to me that it was more dangerous in the sense that if speeds falls to much, you risk a tailspin, and that is never good when the ground is close...

@kebs Slipping does reduce the stall margin (asymmetrically: the outer wing will stall first), but that's why we train for stall recognition.  Re: spins, remember a slip is spin-resistant (the rudder is pushing the nose opposite the direction it would want to rotate) - the stall will be asymmetric and likely not as "pleasant" as one entered in coordinated flight, but a spin is less likely. (Skids on the other hand are spin-prone: The rudder is encouraging the nose to go in the direction it would want to rotate, so when you stall the spin develops quickly.)

@voretaq7 Thanks for that clarification, I indeed mixed up skids and slips!

"I have always felt better crabbing on final until I read about some CFI's really emphasizing a stabilized approach and using that as a justification for slipping on final." -- +1 on that idea.  Only by slipping can you set up a stabilized approach with no need for big changes near the ground.  If you want to wait till the last hundred feet or so to set up the slip, fine, but I don't see the point of big control inputs right next to the ground ("kicking out the crab") unless your aircraft configuration is such that a wing-down slip is not feasible near the ground.  Besides, slips are FUN.

Neither technique is "preferable" - the assumption that one is superior to the other is one of the most common false dichotomies (and over-simplifications) in aviation.
With some notable exceptions (like the Ercoupe which is incapable of a slip and lands crabbed) a proper crosswind landing should involve elements of both techniques.

Crabbing
Crabbing involves turning the nose into the wind so that some component of the aircraft's thrust is counteracting the crosswind, allowing the aircraft's ground track to align with the runway.
Advantages
Flying on final it is advantageous to crab into the wind: It is aerodynamically more favorable than a slip, and as the aircraft is kept coordinated it is more comfortable for passengers than a slip.
Pilots often find a crab subjectively "easier" to maintain for extended periods of time compared to a slip: It does not require the pilot to maintain "crossed controls", and it's similar to how we maintain our course in other phases of flight (you were probably crabbing into the wind on downwind just a few seconds ago, and on instrument approaches flying the localizer in a crab is analogous to tracking a VOR on the enroute portion of your flight).
Disadvantages
On touchdown however landing in a crab will put a side-load on the landing gear (the aircraft is traveling along the runway centerline, but the gear is not aligned with that direction of travel). In extreme cases this can damage the tires, cause difficulty controlling the aircraft, and possibly even damage the landing gear assembly itself.
"Kicking out the crab" to align the landing gear with the runway requires an abrupt control movement at low altitude (generally not a good thing), and if done too early the crosswind will begin to blow the aircraft across the runway (leading to a landing that's not on the centerline, a side-load on the gear, and in the worst case an unintended departure from the runway surface.

Slipping
Slipping involves banking the aircraft so that some portion of the wing's lift is counteracting the crosswind. Ordinarily the aircraft would tend to turn in the direction of the bank, so opposite rudder is applied to prevent the aircraft from turning and maintain the ground track parallel to the runway.
Advantages
Touching down on the runway a slip has the advantage of allowing you to align your landing gear with your direction of travel (down the runway) and avoid side-loading the gear. It also allows you to touch down on the upwind landing gear first which helps to stabilize the early roll-out, and positions your ailerons appropriately for taxiing with the crosswind.
Disadvantages
Slipping down final however is likely to make your passengers uncomfortable - the plane is slanted in one direction, the rudder is pushing the nose around in the other direction, and folks start to feel like their butt is sliding out of the seat.
Aside from passenger comfort a slip is a terribly inefficient aerodynamic arrangement (you're essentially trying to fly sideways), and not a "normal" way to fly in other phases of flight. You may have difficulty estimating the aircraft's performance and require power to maintain your glide path in this configuration.

Combining the techniques
So a crab is advantageous on final (and flying an instrument approach), and a slip is advantageous on touchdown. It logically follows that the two techniques can be combined to provide the advantages of both:

Crab down final (or on the localizer).
At some point transition into a slip.
Exactly where you make this transition is pilot preference, but the transition should be smooth, simultaneously bringing the nose around to align with the runway while banking into the wind to counteract the crosswind component.
Touch down in the slip.
Complete your rollout with the appropriate crosswind aileron deflection, controlling your ground track with the rudder as needed to prevent weathervaning.

Which one is preferred is on occasion part of the aircraft operating limitations.  An example that comes to mind is that crews were trained to land the F-104 in a crab, no slip at all...  Apparently it was safer to abuse the landing gear than to try to slip that particular airplane.

@BrianKnoblauch Aircraft limitations are something else that needs to be considered. As we discussed in chat another aircraft where a slip needs to be limited or avoided is the Tiger Moth as aggressive slips could cause the low-wing to scrape.

In addition B-52 can crab all the way to landing

On point 1, thrust is not counteracting the crosswind, but rather the aircraft's forward speed.  If this were true, a glider could not compensate by crabbing.  ;-)

@Lnafziger Very true, I'm guilty of my own oversimplification there! (For the sake of the overly simplistic "Lift/Weight Thrust/Drag" diagram the vector we're borrowing the correcting force from is "Thrust", but you can get your velocity from other places too and if you have no engine you obviously have no thrust.)

@BrianKnoblauch: Re the "apparently" on the F-104, that thing was notorious for its hostile stall behaviour. So, yeah... ;-)

Aircraft landing configuration also dictate technique. Newer single engine Cessna aircraft prohibit slips with flaps extended; of course flaps are generally frowned on in strong crosswinds but may be necessary if a short field landing is required.

Yes, you can use thrust to counteract a cross wind! Power control is critical in any approach.  Yes, gliders have thrust! They get it using altitude as fuel.  I feel badly for newer single engine Cessna pilots, soon they'll be told that leaving the ground under ANY circumstances is UNSAFE.

One disadvantage of a slip that you didn't mention is that it can't be used for aircraft with low-clearance underwing engines, because the engine on the low wing would hit the ground and fall off.

It seems airbus recommends a crab:

IV Final Approach Technique Figure 1  shows that  depending on the recommendations  published in  the aircraftoperating manual, the final approach under  crosswind conditions may be conducted:

With  wings-level  (i.e., applying a drift correction in  order to track the runway centerline,  this type of  approach  is called a crabbed approach  [Airbus recommended technique]),

or

With a  steady sideslip  (i.e.,  with the aircraft fuselage aligned with  the runway centerline,  using a combination  of  into-wind  aileron  and opposite  rudder to  correct the drift).

Airframe manufacturers  consider the following factors  when recommending a wingslevel or a steady-side-slip approach:

Aircraft geometry (i.e.,  pitch attitude  and bank  angle limits for preventing tail  strike, engine nacelle contact or  wingtip contact)

Ailerons (roll)  and rudder (yaw) authority

Crosswind  component.

This Flight  Operations  Briefing  Note  focus on  the  wings-level /  crabbed approach technique,  recommended by Airbus, to discuss the associated flare and decrab techniques  depending on  the crosswind component.

From AirbusFlight Operations Briefing Note, Landing Techniques, Crosswind Landings pdf

Airliners have to crab because if they use sideslip (i.e. wing down) the engines could touch the runway. Many airliners can touch down in a crab, i.e. no need to kick off the crab on some types e.g. B747 (an airline pilot may be able to expand on this).

@Philip Johnson - very true... a B747 documentary on the first flight testing of the B747 said the gear was approved for contact with the ground up to 45deg and they showed a test of it !!!

@jwzumwalt - 45 degrees is amazing for such a heavy aircraft. Try that in a cessna it wouldn't end well!

I saw a B747 land at the old Hong Kong airport hit pretty close to 45deg, bounce... then the pilot swung the nose straight, it was blown off center some what dangerously, he planted it again at a lesser angle, and there was smoke from rubber all over the place from each tire. There was so much smoke from rubber you could clearly see the tip vertices!

I was taught the same way as Hmb.  Although the larger Part 121 planes must touch down pretty level to avoid dragging an engine or a wing, they fly crabbed until short final.  I was told that this is for passenger comfort as flying un-coordinated can make some squeamish pax a bit green.  So for me, it is crab and kick - fly the approach crabbed and kick out the crab before you touch down.

To answer this
It depend on what are you flying.
From my side as an airbus pilot we are trained always to approach with a crab then at the flare we de crab and allign the nose with the runway centerline using rudder.

Hi Hmb, you could provide some good perspective if you expand on these points and give more background/explanation.

I up voted Dave's answer, but mine is the same but opposite. I learned to slip all the way down final, and I prefer it, but I think it depends, like was mentioned in the other answer.
I like to slip because it aides in bleeding off speed, I don't have to upset the controls of the aircraft at the last minute, and I'm keeping the upwind wing down throughout the final, minimizing the chance of a gust lifting the wing up unexpectedly.
I think it depends on the aircraft, pilot, and situation, but I'm comfortable with the slip and that's what matters to me.

You're comfortable with risking an engine strike?

This comes down to situation and personal preference. I am more preferential to the crabbed approach as that is how I was taught and that is what I am comfortable with. If you are talking strictly from a safety point one could argue that what ever method you are more proficient and comfortable in is "safer". With a slipped approach you are coming in along the runway path so you will most likely not need much last second correction. A crabbed approach will require some adjustments close to touch down but like any landing you should be on full attention.
From a technical stand point, a slipped approach puts the plane in uncoordinated flight which can be viewed as inefficient. Slipping also increases drag that you will need to compensate for with power.
I also find that I prefer to crab in really gusty or high wind situations to keep the plane a bit more stable on final.

I prefer crabbing down to round out and touch down, while I am crabbing I have all my controls in their neutral position with full control authority available, slipping requires deflection  away from neutral which limits control action available while decending through possible shear or surface feature induced turbulence

Welcome to Aviation Stack Exchange!  You could make your answer better by providing sources to back up your claims.


text:
Does ATC have to maintain separation on diverging aircraft?
Say you have an eastbound flight at FL210 that wants to climb and a westbound flight on a converging track at FL220 that wants to descend. There's the minimum required 1000 ft vertical separation. Once the planes have passed each other can ATC clear them to climb/descend right away or do they have to wait until they are 5nm apart before they can break the vertical separation? And what about TCAS? Would it consider that a conflict?

TCAS is based on closure rate, so it shouldn't trigger. I have no idea what is legal for the ATC though.

"Traffic in sight. Will maintain visual separation" is also good enough to break separation minima

Not in Class A airspace, @rbp.

Yes, this is allowed, subject to certain conditions. See FAA Order 7110.65 (the ATC "bible"), section 5-5-7, for the full details.

...all other approved separation may be
  discontinued and passing or diverging separation applied when ... Aircraft are on opposite/reciprocal courses and you have observed that they have passed each other; or aircraft are on same or crossing courses/assigned radar vectors and one aircraft has crossed the projected course of the other, and the angular difference between their courses/assigned radar vectors is at least 15 degrees.

That's in the terminal environment; it's still possible in the enroute environment but a bit more restrictive due to the greater aircraft speeds and poorer radar accuracy.

It depends wether you get procedural or radar service. In case of procedural service if both pilots report clear of traffic (beeing previously informed of the traffic) the longitudinal sepparation criteria is met.
In case of radar service longitudinal sepparation minimas can vary from 5 to 10 NM or more.

In my experience it's moot. Usually when you observe that the tracks have crossed, by the time you issue the clearance to climb/descend the acft are 5 miles apart anyway due to speeds at those altitudes.
Cavver - Interesting that I've never heard it referred to as "procedural" separation before! Is that a non-us reference? We always called it manual separation or non-radar. It was always a challenge to try and set off the immediate alert buzzer at the desk and have a manual rule to invoke. Made management grumble :-D

I've heard it referred to as procedural control also, but when there was no radar available, like over oceans or in countries that don't have much radar control.

I've always called this "the bubble"

Also, asking a question from another answerer should go in the comments below their question. You should have that feature available now, it only takes 50 rep. But unless it's under their question you need to put an @ in front of their username to get their attention. I.e.: @Iain

It's great to have some more knowledge from someone with ATC experience here!

You will have to comply with the minimum horizontal separation required by the regulations before crossing their levels.
TCAS might consider them clear of conflict before they have the minimum horizontal separation required (3, 5, 8Nm, whatever that may be), but remember TCAS is not a way of providing ATC separation.


text:
Why could these aircraft not prevent total hydraulic failure?
Then have been three incidents (UAL232, JAL123 and DHL 00-DLL) where damage to an aircraft resulted in complete loss of hydraulics. Only one of them managed to make it down safely using differential thrust. Each plane was from a different manufacturer. All three aircraft had triple or quadruple redundant systems. This seems like plenty of redundancy, but the Achilles heel is that, regardless of how the systems are piped around the aircraft they all have to converge at each of the control surfaces.
My question is why did damage in one part of each hydraulic system result in loss of the entire system? For example in the DHL incident the left wingtip was damaged with all three lines. Is there not a way to isolate that wing and cut off hydraulic flow to protect the integrity of the rest of the system? You would lose control surfaces on that wing, but still have elevator, rudder and the other wing. Do any aircraft have a way of preventing this?

There is no general answer, but the closest one would be bad luck.
In the DC-10-10, the aircraft of UAL232, all three hydraulic lines went through one hole in the tail bulkhead which was damaged when the engine failed. This is no real redundancy, and it was bad luck that this particular spot had been hit.
In the Lockheed Tristar, the same accident would had damaged probably just one line. There, the three hydraulic lines are separated as much as possible in the tail section (one on top, one on the left side and one of the right side of the fuselage). Lockheed has more experience with military design than Douglas, so the Lockheed engineers were more aware of redundancy issues.
In the A300 involved in the Baghdad attempted shoot-down incident, the same bad luck happened. Normally, each aileron is powered by two out of three hydraulic circuits, so one would remain if the other two are hit. Note that the green and yellow circuits were lost immediately, while the blue circuit failed only 20 seconds later. But in this case the damage extended to the spoilers, so all three circuits were affected in the end. There is only so much damage you can protect against.
Also for JAL123 the damage was just too extensive to not involve all four hydraulic circuits. The rear pressure bulkhead burst and blew most of the vertical tail surface away. It can be argued that the real culprit, however, was the chief pilot's refusal to don his oxygen mask. But the aircraft was only marginally stable and very hard to control.
Hydraulic fluid needs to be pumped continuously to heat it so its viscosity stays low. If you add shutoff valves, this pumping could be impeded. Note that the spoilers of the Baghdad A300 were not sucked open because non-return valves blocked the lines to the spoilers, helping in keeping the aircraft flyable.
If you put shutoff valves along the hydraulic lines, you open up a new avenue for failures. The idea of hydraulics is that you use just one pump and can create the maximum force at any point, since rarely would you need to move several actuators at maximum load in parallel. Splitting up the circuitry to enable the pilot to isolate parts of the airplane will take away this advantage, resulting in a more complex, heavier hydraulic system. And who knows if those added parts would not malfunction or be mishandled, resulting in a new class of entirely avoidable accidents.

+1 and I think the last sentence is crucial. You increase safety by designing new safety measures only until these new stuff's failure is more problematic than whatever you had before.

The idea of hydraulics is that you use just one pump  does this mean there's only one pump for all lines or one pump for each system (blue, green, yellow)?
Would it be possible, especially on 4 system designs, to have one system not go to all control surfaces? For example, say yellow system goes everywhere except the left wing. Green system does not go to the tail section. That way you still have triple redundancy to each area, but if you get damage on the left wing, like DHL, you lose all systems * except* for yellow. You would lose control to the left wing but you would still have one system to the rest of the plane.

@TomMcW As a general rule, the flight controls are the most crucial, hydraulically-powered things on the aircraft.  So you may have multiple hydraulic systems, powering different things, but ALL of them power the flight controls, in the hopes that losing some systems still leave you with functioning flight controls.  The F-16 has two, separate hydraulic systems.  Both power the flight controls.  Damage to one means the flight controls still work, even if the other systems have problems.  Separate pumps, separate lines, routed separately.  But some things you just can't plan around, like JAL123

"In the Lockheed Tristar, the same accident would had damaged probably just one line."  Also, the Tristar has four hydraulic systems, rather than just three.

@Sean: Also in the tail? So far, I thought that there were only three lines leading to the tail surfaces. Please help me with the details!


text:
How common are regulations limiting when the FO can be PF?
I was reading that in India regulators have specified certain airports at which the FO is not allowed to take off or land.

The Pilot-in-Command shall not authorise a Co-pilot to effect supervised take-off or landing when TODA/LDA is  marginal. A margin of at least 1000 feet must be ensured.  Critical airfields,  [Mangalore, Port Blair, Leh, Agartala, Lengpui, Shimla & Kullu airports]  are specifically excluded for supervised take off and landing.

As a matter of fact they have several limits in place where 'supervised takeoffs and landings' are not allowed and the PIC must fly the plane.
In this unique to India or are limits on FO's common?

@mins I'm not sure how else one could interpret that. The whole document is on what a co-pilot can and cannot do. That sentence removes the PIC's authority to decide otherwise.

After second thought, I agree with your interpretation.

All the airports listed in the AIC are quite tricky to takeoff/land in. For example, the Lengpui airport runway is a classic example of tabletop runway.

Source: www.misual.com
The Leh runway is surrounded by steep hills.

Source: geolocation.ws
The kullu airport runway has a river in both its ends.

Source: www.airportsindia.org.in
This is the reason for these restrictions. I'm not aware of any such regulatory restrictions in case of other countries. However, the operating airlines can have their own rules about (Category C) airports.

Yeesh, Kullu not only has a river at both ends, but all those buildings (residences???) just off the right-hand end, as well!

The old Kai Tak was another good example of what would be a Category C airport, with its insane approach procedure; Aspen-Pitkin is another great example of an airport that is this way.  Don't forget to mention that the FO often can't be PF on the ground -- many Boeings only have one tiller!

I found out about the regulations while reading about the crash of Air India Express flight 812 where the captain overran the tabletop runway and slid the 737 over the cliff. I've never head of a rule requiring the PIC to land at a difficult airport. That seems arbitrary to me.

I know the captain will almost always be more experienced than the FO, but circumstances may not make him the best choice. It may be his first time flying into that airport whereas the FO may be very familiar with it. One might even make a case that it's better to have the FO flying. In the case of that crash the captain made a poor decision to continue an unstable landing while the FO kept suggesting they go around. If the opposite were true and the FO was flying a bad approach the captain could order a go-around

@unrecognizedfallingobject But that's just a limitation of the aircraft. No one is saying the FO is not allowed to do that, it's just that Boeing didn't see a need to provide two of them.

@TomMcW -- you could say that Boeing is saying that the FO shall not taxi the plane!

The FAA has restrictions for second in commands when conducting certain operations under Part 135 (notably, landing when more than 60% of the runway is required) and the SIC has less than 100 hours in type specified in 14 CFR 135.4(a)(3):

(3) Pilot operating limitations. If the second in command of a
  fixed-wing aircraft has fewer than 100 hours of flight time as second
  in command flying in the aircraft make and model and, if a type rating
  is required, in the type aircraft being flown, and the pilot in
  command is not an appropriately qualified check pilot, the pilot in
  command shall make all takeoffs and landings in any of the following
  situations:
(i) Landings at the destination airport when a Destination
  Airport Analysis is required by§ 135.385(f); and
(ii) In any of the
  following conditions:
(A) The prevailing visibility for the airport is
  at or below3/4 mile.
(B) The runway visual range for the runway to be
  used is at or below 4,000 feet.
(C) The runway to be used has water,
  snow, slush, ice, or similar contamination that may adversely affect
  aircraft performance.
(D) The braking action on the runway to be used
  is reported to be less than “good.”
(E) The crosswind component for
  the runway to be used is in excess of 15 knots.
(F) Windshear is
  reported in the vicinity of the airport.
(G) Any other condition in
  which the pilot in command determines it to be prudent to exercise the
  pilot in command's authority.


text:
Why does the A320 right aileron float on dual engine failure?
On this question for the A320 after dual engine failure it says:

When the hydraulic power is lost, the right aileron is lost, and is in the upfloat position.

Why would this affect only the right aileron?


text:
Is precession a problem for two-bladed propellers?
The vast majority of wind turbines are a three-bladed design. Although two blades would be more efficient, two-bladed designs are more likely to suffer from precession. Do two-bladed propellers suffer from precession? If not, why would it be a problem for wind turbines, but not aircraft props?

Wind turbines suffer not from precession per se, but from precession 'wobbling', when the gyroscopic force varies dramatically over a revolution. Normally, a 3-bladed prop would be heavier than a 2-bladed one, and will have greater gyroscopic effect (at the same RPM). However, it will be better balanced across a revolution. The closer to a solid disk, the better.
(In fact, a 3-bladed prop appears to be perfectly balanced in extreme positions. Considering blades slender rods or length $r$, the moment of inertia (MOI) of the propeller disk for turning across the spin axis (e.g. yawing) will be $1/3\cdot mr^2 + 2\times 1/3\cdot m(r\cos60°)^2 = 1/2\cdot mr^2$ in one extreme position (like −<), and $2\times 1/3\cdot m(r\cos30°)^2 = 1/2\cdot mr^2$ in the Y position. Compare this with variation from almost 0 to $2\times 1/3\cdot mr^2$ for a 2-bladed prop).
For a 2-bladed prop, this wobbling occurs at double the frequency of rotation. For wind turbines with their ~20 RPM this is < 1 Hz, a frequency that can interfere with normal yawing and even damage the structure. But aircraft props turn at much higher speed; a typical 2400 RPM will give you an 80 Hz hum. May be unpleasant, but far away from the natural frequencies of manoeuvring and of the airframe.
Also important to note that there are two effects at play here: precession (gyroscopic effect) exerts a force perpendicular to the applied rotation, but there is also simply a moment of inertia of the prop/rotor across its spin axis (as calculated above) that needs to be overcome. This latter one applies even if the rotor is stopped, but both wobble when it spins. Now, for airplanes with their fast spinning but relatively small props, the gyroscopic effect may be noticeable (see below), but just MOI is totally negligible (the mass of the blades is nothing compared to the rest). Yet for wind turbines the opposite must be true: the rotor mass/MOI is (probably) the dominant mass of the whole thing.
As for precession itself, i.e. the tendency to turn perpendicular to where you want to turn, most aircraft don't 'suffer' from it either. Aircraft simply don't turn (manoeuvre) too fast for it to be a problem, and the aerodynamic forces outweigh the gyroscopic ones by orders of magnitude. On normal propeller aircraft, only if you want to notice it, you might notice it when doing a manoeuvre.
In some circumstances, however, precession can be quite significant. This involves the cases when angular velocities are high (aerobatics), the aerodynamic forces are low or very finely balanced (hovercraft), or both (spin).

I could probably word my question better, but you seem to have honed in on what I’m asking about, specifically the “wobble.” I think the precession effect others are referring to is a different, but related, phenomenon.

So, the concern in the wobbling is when the frequency of the wobble is low enough to interact with other natural motions? Does the number of blades affect the frequency of the precession?

@TomMcW, I'd say yes, although there may be higher-frequency modes (particularly aeroelastic ones) that can always cause problems. But in practice I'm not aware of any with regards to this phenomenon on airplanes. (Two-blade helicopters may be susceptible, worth checking). The wobbling occurs at a frequency with which the mass configuration repeats itself over the revolution, i.e. it grows proportionally to the blade number for a given RPM. But, in many cases greater number of blades means lower RPM.

@TomMcW, I updated the answer with MOI calculation to demonstrate how much better 3 blades are, and added the simple (non-gyroscopic) MOI effect that must be significant for wind turbines but not for airplanes.

Yes, two blades propellers will produce precession effects during maneuvering, just like any other propeller will.  It is barely noticeable for normal operations.  It is quite noticeable during hard maneuvering or aerobatic flight and coordinated use of rudder and elevator pressure is necessary while maneuvering in these regimes to counteract this effect.

This question leads to a consideration of precession induced whirl-mode phenomena, irrespective of number of blades. The turbine mounting structure may be of sufficient strength, but insufficient stiffness. Many lives were lost in the early days of large turboprop airliners (Having 4 blade props) due to resonant, whirl-mode, structural failure. In the case of the Lockheed L-188 Electra, all it would take is an asymmetric gust to get it started. Due to reasons stated in previous answers, number of blades would not be of primary concern.

A one-bladed propeller introduces the least drag, but presents elementary balance issues. Two bladed propellers suffer from worse precession than three bladed propellers, and in a turbine that shows up with the blades are in a horizontal position and the turbine is pivoting / yawing due to a wind change. A three-bladed propeller balances some of that force and ends up being the best compromise tradeoff between So Many Blades and air resistance.
More depth here: http://explorecuriocite.org/Explorer/ArticleId/193/why-dont-wind-turbines-have-more-than-3-blades-193.aspx

Interesting article, but this post rather re-phrases the question than gives an answer. Yes, 3 blades are better for this phenomenon, so how do aircraft cope with 2?

Yes, gyroscopic precession is a thing on two-bladed airplane propellers. I fly a C-172 and it is listed as one of the factors in the airplane's left turning tendencies.

Gyroscopic precession is a thing of any rotating mass. The heavier the mass (the prop), and the faster it spins, the greater the force. The question is, apparently, about a different thing: the vibration of this force. Besides, precession is not part of the 'normal' left turning tendency (which is caused purely by aerodynamic forces); precession happens when there is a rotation other than about the spinning axis. For C172, it will yaw you left if you quickly pitch down, and yaw right if you pitch up. (And pitch you up if you push the left pedal, etc.)

Precession is a gyroscopic effect, and –for the same angular speed– the forces involved depend on the moment of inertia of the propeller, that (for the same blade configuration and materials) is proportional to the number of blades.


text:
How does the airflow around winglets reduce drag?
I am familiar with the basic aerodynamics of wingtip vortex generation and I know that winglets reduce drag by reducing wingtip vortex generation.  I have never seen any illustration of exactly how winglets interact with wingtip vortices to do this.  I have also heard there is a small thrust component to the winglet's lift but am also unable to visualize this as well.  The multiple designs in use are confusing.  Why not just curve the end of the wing down to inhibit the vortex from spilling upwards?

This website about 737s has a good explanation: http://www.b737.org.uk/winglets.htm

This video perfectly explains what happens, except it doesn't use winglets, but different shape of the wings, but the idea is the same:
https://www.youtube.com/watch?v=CnvIf3vFEYA
What the video describes is when wings don't have winglets, air flowing under the tip of the wing escapes to the side of the wing and upwards, creating a vortex which reduces lift. Addition of winglet reduces the vortex and thus increases lift.
In the video however, new type of wing is shows that doesn't require winglets all together.

It would be better if you could reassume the video in the post, the video could disappear at any time. If you can't, this would be better as a comment, not an answer.

Edited with more info.